16 APPLICATION OF THE LAWS OF HYDRAULICS
building is always greater than it is on the lower floors. The
increase of the pressure in the case of gas is the inverse of that
which occurs with water.
In Hutte, in the chapter upon illuminating gas, the following
statement occurs: " A rise (or a drop) of 10 m in height in the
mains corresponds to an increase (or a loss) in pressure in the
neighborhood of 7 mm of water column." <X)
The following rule, to be followed in the location of a gas
works is given in order that there may be a uniform pressure in
the holder and throughout the distributing system: " The gas
works, should be installed at the lowest point in the system and
should be so located that the highest point in the supply system
is furthest from the works, and the lower parts are closer to the
works; under such conditions the loss of gas pressure due to
friction in the mains is compensated for by the increase of the
hydrostatic pressure in the piping, in proportion to its distance
from the works and its elevation above their level/7
The existence of hydrostatic pressure in the gas having been
established, it is now possible to take into exact account those
phenomena which are presented in metallurgy.
(a) The Draft Fallacy of Metallurgical Furnaces.—It was
believed for a long time that furnaces having a natural current
of air through them operated through the effect of their chimneys.
This is an error due to a poor interpretation of the facts. One
of the first principles which must never be forgotten is that the
only furnaces which operate by the draft provided by the chimney
are those which have no working doors or openings for the charging
of material to be heated. Furnaces of this sort include boilers,
entirely enclosed in a setting built of brick or other material,
crucible melting furnaces which are very nearly hermetically closed
up, iron tube air heaters (formerly used for heating the air at
blast furnaces), the Cowper hot blast stove, etc.
Furnaces which have working doors to their laboratories or
heating chambers, such as reverberatory furnaces, melting fur-
naces, puddling furnaces, brick kilns, Siemens furnaces of all
kinds (open-hearth and others), do not operate by the draft of the
chimney. The chimney connected with these furnaces is only an
apparatus for removing the products of combustion from the
laboratory. The colossal chimneys which are very often seen are
C1) Vol. II, p. 869, French edition of 1911.